An indentation method is used to study the variations in Young's modulus, hardness and fracture toughness of air plasma-sprayed thermal barrier coatings at a high temperature. The coatings were exposed to 1100 degrees C during 1700h. A sudden increase in Young's modulus for the first 600h was observed, while the hardness increased after 800h as a consequence of sintering. Conversely, there was a reduction of 25% in fracture toughness after 1700h, evidencing the thermal barrier coating degradation. The evolution of these mechanical properties was correlated with microstructural changes. After 1700h, the thermally grown oxide thickness reached 6.8m, the volumetric percentage of porosity was reduced from 6.8% to 4.7% and the amount of monoclinic phase increased to 23.4wt%. These characteristics are closely related to the stress distribution in the top coat, which promotes cracks nucleation and propagation, compromising the coating durability.